Reijo Ruotsalainen started his career in Kärpät. From his early days, "Reksa" was recognized as a talented defenseman while playing for Kärpät Junior teams.

Kärpät, along with Ruotsalainen, got promoted to SM-liiga from 1. Divisioona for the 1977–78 season, and Ruotsalainen played 30 games and posted 23 points in his first SM-liiga season.

During the 1979–80 season, Ruotsalainen and Kärpät played in the SM-liiga Playoffs, but were eliminated from the Finals by HIFK. Kärpät, however, won the bronze medal series, 2-0, against TPS, and Ruotsalainen along with Kärpät got to enjoy their first medals in SM-liiga history.

The 1980–81 season proved to be a breakthrough for Ruotsalainen. Kärpät broke their way through to finals against Tappara. Before the fifth and decisive game, the series was tied 2 games to 2. After the first period, Tappara was leading 2-0 but Kärpät made a spectacular comeback and won the game 5-2.

Ruotsalainen was signed by the New York Rangers of the NHL in 1981. During his debut season, Ruotsalainen scored 56 points in 78 games.

Ruotsalainen, initially paired on defense with Barry Beck, was eventually moved to forward. His Head Coach Herb Brooks said that Ruotsalainen was the best skater in NHL. Ruotsalainen's fluid skating helped him compensate for his lack of size (Ruotsalainen was 1,73m tall).

Ruotsalainen had his NHL-career high in points during the 1984–85 NHL season, when he scored 73 points in 80 games and led the Rangers in scoring. Ruotsalainen played in the 1986 NHL All-Star game.

Ruotsalainen's contract expired after the conclusion of the 1985–86 NHL season. Ruotsalainen was without a team before the start of the 1986–87 NHL season, and accepted an offer from SC Bern, a Swiss team that played in Swiss Nationalliga A.

Ruotsalainen played in 35 games and scored 53 points for SC Bern before returning to the NHL.

Ruotsalainen was acquired by SM-liiga team Tappara for reminder of 1993–94 season. Ruotsalainen played six regular season games with Tappara and scored 6 points. Ruotsalainen played in the SM-liiga Playoffs with 3 points in 9 games. Tappara however did not have any success in the playoffs and they were eliminated by TPS in Semifinals.

After his season in Zürich, Ruotsalainen decided to end his career in Finland. Ruotsalainen played for Kärpät in the Finnish 1. Divisioona for two more seasons, scoring 31 points in 44 games during his Final season.